(a) Field of the Invention
The present invention relates to an electrochemical element, specifically an electrochemical element with improved energy density comprising multiply stacked electrochemical cells.
(b) Description of the Related Art
There has been growing interest on energy storage technology. The applicable field of the battery has been expanded to cellular phones, camcorders and notebook computers with recent addition of electric vehicles into this list. Such expansion has led to increased research and development of batteries with visible outcomes. In this respect, researches on electrochemical elements are one of the fields that have been receiving much attention, among which rechargeable battery is the central field of interest. Recent developments have turned its way to designing new batteries and electrodes to improve capacity and specific energy.
Among the secondary batteries being used, lithium ion battery developed in the 1990s has become increasingly popular because it has higher operating voltage and energy density compared to Ni-MH, Ni—Cd, and sulfuric acid-lead batteries that use aqueous solution electrolyte. These lithium ion batteries, however, have safety problems resulting from the use of organic electrolyte, which causes the batteries to be flammable and explosive. Also, lithium ion has the weakness of having difficult manufacturing process. Recent lithium ion polymer batteries have overcome such shortcomings of the lithium ion batteries and are anticipated to become the batteries of the next generation. These lithium ion polymer batteries, however, have relatively low capacity compared to lithium ion batteries and have especially insufficient discharging capacity at low temperatures; and thus, need to be improved.
The capacity of the batteries is in proportion to the amount of the electrode active materials. Thus, it is extremely important to design a cell structure that can be filled with as much quantities of electrode materials as possible within the limited space of the battery package. The most widely known and used type of cell structure is a jellyroll shaped structure used in a cylindrical or a prismatic battery. Such a structure is prepared by a process of coating and pressing active electrode material onto a metal foil which is used as a current collector, followed by cutting it into a shape of a strip having predetermined width and length, and then separating the anode and cathode using the separator film, and then winding it into a spiral form. Such a jellyroll structure is widely used for manufacturing cylindrical batteries. This structure, however, has small radius of curvature at the center portion of the spiral, which often results in extreme stresses at the bending surface of the electrode, often causing exfoliation of the electrode. This facilitates the deposition of lithium metal at the center portion of the electrode during the repeated charge and discharge of the battery, which may shorten the lifespan of the battery while degrading the safety of the battery.
Generally, the widely known and used method of manufacturing a thin prismatic shaped battery comprises aforesaid process of winding the spiral shaped jelly roll into an oval shape and then compressing it, followed by inserting it into a rectangular container. This method is not free from aforesaid problems of reduced lifespan and safety, but rather has increased problems caused by the decrease in the radius of curvature due to the oval shape. Also, the problem of reduced performance is greater because manufacturing a tight spiral structure is inherently impossible. Furthermore, discrepancy of the oval shape of the jelly role and the rectangular shape of the container reduces the rate of utilized volume. This is known to reduce approximately 20% of the weight energy density and 25% of the volume energy density when the container is taken into account. In reality, a prismatic lithium ion battery is reported to have lower capacity density and specific energy compared to a cylindrical one.
Recently, various patents and technologies proposing to solve the problems of the spiral jelly roll type structure and providing cell structures suitable for a prismatic container are being published. These proposals, however, only provides partial solution to the problems or causes other problems more difficult to solve so that they have not become a practical solution. For example, U.S. Pat. No. 5,552,239 describes a process of first placing and laminating a separator layer or polymer electrolyte between the cathode and anode, then cutting it into a form of a strip with predetermined length and width, followed by gradually folding a cell having an anode/separator layer/cathode layered structure into a square form. The inventors of the present invention have tried to replicate such a process but have found out that it was difficult to manufacture the cells for such a use. The laminated cells were so stiff that it was difficult to fold and when it was folded by exerting force, the problem arose in the folded area because it was fractured in a manner similar to the jellyroll typed cells.
In fan-folding method described in U.S. Pat. No. 5,300,373, the pressure and stresses at the inner layer of the abruptly bending portion are transferred to the outer layer and diverged so that twisting and stretching occur, finally resulting in a “dog bone” shaped cell. Thus, the problems of exfoliations, cracks, crumbles or snapping, encountered in jelly role type structure also occur frequently. Also, the cells with this structure are inherently prone to snapping; and therefore, the possibility of making a practically applicable battery is very low.
Meanwhile, U.S. Pat. No. 5,498,489 attempted to solve and improve such problems in the bending portions. It provides a fundamental way of avoiding exfoliation of the electrodes by leaving out the electrodes at the folding portions and providing connections only through the use of current collectors and separator layers or polymer electrolyte portions. But, there is difficulty in composing such a cell. Furthermore, too much current collectors are used and the structure wastes too much electrolyte. Thus, the structure is not very practical because it has many inefficient factors.